Mechanism of the Formation of Pores in Starch

Starch Granules Are Not Destroyed

Hydrolysis

Porous Starch (PS) can be produced by reacting various raw starches with starch-hydrolyzing enzymes under the gelatinization temperature of starch. Using this method, the starch granules are not destroyed, although pits/craters on their surface and/or extensive interior erosion develop due to hydrolysis of the amorphous regions of starch.

• Introduction to Porous Starch (PS)
• Porous Starch Preparation
• Porous Starch Structure and Properties
• Application of Porous Starch
• Porous Starch Wastewater Treatment

Porous Starch (PS) can be generated based on α-amylase and glucoamylase used as hydrolysis enzymes. The microstructure of the starch granules was greatly dependent on the enzymatic treatment, but in both cases PS granules were obtained. α-Amylase produced some holes in the starch granules, and its action was not dependent on granule size. Glucosidase was considerably active on starch granules resulting in perforated granules. α-Amylase is an incision enzyme, which can randomly hydrolyze α-1,4 glycosidic bonds of starch, and glucoamylase is an excision enzyme, which can not only hydrolyze α-1,4 glycosidic bonds but also α-1,6 glycosidic bonds. The pores are formed when α-amylase and glucoamylase degrade the amorphous regions of the starch granule. However, as the hydrolysis rate of the glucoamylase is low, a single enzyme type cannot hydrolyze starch efficiently. With an increasing amount of enzyme, the hydrolysis ratio increases continuously, as more enzyme molecules combine with starch molecules. When a small quantity of enzyme was added, small and shallow pores were formed in the starch, and more pores were formed only when the enzyme ratio increased. These changes were more obvious on the A-type (large) granules, and the equatorial zone of the granules (the middle section) was much more susceptible to the enzyme than other parts of the granule due to the weaker structure.

As starch is a solid granular powder and insoluble in water, a certain volume of solvent is required to ensure that starch granules are uniformly dispersed in the reaction system and efficiently contact with the enzyme. Only when V_H2O/m_starch increased to the extent that the enzymatic reaction was carried out efficiently, numerous pores were formed, and a larger specific surface area was generated.

Temperature is another important restrictive factor which affects pore formation. α-Amylase and glucoamylase have less activity when the environmental temperature is low. When the temperature reaches a suitable range for the enzyme (about 50 °C), the reaction efficiency increases. When the temperature is increased to 55 °C, an increase in the hydrolysis ratio and a decrease in the adsorption ratio are observed. The reason for this is that the reaction temperature is required to reach the gelatinization temperature of starch, and gelatinization then occurs. The hydrogen bonds between starch chains are broken, and the crystalline region is damaged. Therefore, the hydrolysis ratio significantly increases. However, when the degree of hydrolysis is high, the pores in the starch granules become larger and eventually collapse and break.

However, native starch is often poorly and slowly hydrolyzed in its granular form. Enzymatic treatment is usually combined with other treatments such as sonication. The native starch granules have a smooth surface. However, after sonication, the surface of the granules was uneven, and some grooves and fissures appeared on the exterior of the granules which increased with increased sonication time. High-power ultrasound has an obvious impact on starch granules due to the collapse of cavitation bubbles and high-pressure gradients which mainly affect the amorphous regions of the granules and induce pores and fissures on the granules and change some physicochemical properties of the starch.

Acting as Pore-Forming Agents

Starch can be used as a pore-forming agent, e.g., as an additive in traditional slip casting, or it may be used as a combined pore-forming and body-forming agent (consolidating agent) in so-called starch consolidation or starch consolidation casting. It was proved that the starch powder starts to burn to release CO₂ at ~350 °C and is complete at ~550 °C; thus, pores with a pore size as large as the particle size of used starch (~10 μm) are then expected to form.

In a study of porous Si₃N₄ ceramics produced by the addition of potato starch, bulk density and porosity changed gradually as the starch content increased, while increased porosity did not fit the volume of the starch added. The collapse of starch-originated pores during starch removal may be the reason for these inconsistencies. Porosity was affected by starch content, while the changes were not consistent which may have been caused by different sources of starch. More large pores approximately 10–15 mm in size were formed in the specimen when the corn starch content was increased. During the production of hierarchically porous silica ceramics, a change in the concentration of soluble starch hydrosol can affect the final microstructure of the template, such as macropore size and wall thickness, in the unidirectional solidification process. For hydrosol containing 10 wt.% soluble starch, the pore size was in the range of 3.5–12 μm. Different to the 10 wt.% starch monolith, the 15 wt.% starch material had reduced macropore size and dendritic-like features, with a pore size in the range of 4–9.5 μm. This may have been due to the tip splitting of the local ice dendrite and subsequent entrapment of a small fraction of starch species created by tip healing. Calcination temperature also affected the apparent porosity and flexural strength of porous starch ceramics.

However, starch granules generally change their size as well as their shape due to swelling in the aqueous suspension; therefore, it is difficult to determine the characteristics of the pores of the final porous ceramics. To solve this problem, porous alumina ceramics with ultrahigh porosity were prepared by combining the gel casting process with the pore-forming agent technique. When dissolved in water, starch will absorb water and generate a fibrous structure. This phenomenon has been applied in starch consolidation, in which starch works as a body-forming agent, to connect the ceramic powders together.

Starch Granules Are Destroyed

Porous Starch (PS) can also be prepared when starch granules have been partly or completely destroyed. Guan and Hanna prepared starch foam by extruding a native corn starch/ starch acetate blend in a twin screw extruder, and EI-Tahlawy et al. prepared microcellular foams by reacting starch with alkyl ketene dimer in an alkaline medium, followed by precipitation by solvent exchange.

Physical Activation

A freezing-solvent exchange has been used to prepare Porous Starch (PS) by replacing ice crystals in frozen starch gel with a mixed solvent of ethanol and water. In this method, porous structures were created following replacement of ice crystals in the frozen gel with various ethanol/water solutions. Different porous structures were obtained when ice crystals were replaced with different ethanol/water solutions. Water was immobilized in the gel when the paste was cooled to room temperature and then frozen. Porous structures were created after replacement of ice crystals in the frozen gel with various ethanol/water solutions. The size of the holes in PS varied with different ethanol/water ratios. The products from this method usually exhibit a low degree of crystallinity, which impairs their scope of applications. Microwave vacuum drying is an alternative method to achieve good porosity and other properties. With this method, pores are formed by a pressure difference established between the inside and outside of the tissue due to steam generation. Microwave energy is absorbed directly by physical movement of the material, and rapid drying occurs in a vacuum, along with material generating pores.

Porous Starch (PS) can be prepared via gelatinization by breaking the intrinsic hydrogen bonds within the macromolecular chain of starch, followed by freeze-drying and carbonization. Bao L, Zhu X, Dai H tried to modify starch with mercaptosuccinic acid during the gelatinization of potato starch and after freeze-drying and prepared PS xerogel by inserting mercaptosuccinic acid molecules into the starch chains. The water molecules within the gelatinized starch evaporated gradually during the process of freezedrying, resulting in self-aggregation of the macromolecular chains of starch. Due to the insertion (during gelatinization) and evaporation (during freeze-drying) of water molecules within the starch chains, starch xerogels exhibited a wrinkle-shaped morphology. The appearance of the porous structure was attributed to the fact that mercaptosuccinic acid molecules were introduced into starch to break the intrinsic hydrogen bonds within the starch chains. The entrapped mercaptosuccinic acid molecules were retained in the starch xerogel during the freeze-drying process due to the formation of intermolecular hydrogen bonds between mercaptosuccinic acid and starch. As a result, the self-aggregation of starch chains was hindered due to the existence of mercaptosuccinic acid molecules, leading to the formation of macropores within the obtained xerogels, and the presence of abundant macropores. The aerogel template was removed by thermal treatment to decompose the starch.

Supercritical CO₂ technology was also introduced and promoted as an innovative technique for the production of Porous Starch (PS). Soluble starch was dissolved in water. The water in the gel was then replaced with ethanol, resulting in monolithic starch aerogels after drying using a supercritical CO₂ technique. Reactive supercritical fluid extrusion was also investigated in the cross-linking of starch blends by phosphorylation. A starch blend was mixed with sodium trimetaphosphate and extruded at 60–70 °C with NaOH solution and supercritical CO₂ as a blowing agent to generate starch foams. It is worth mentioning that this process greatly influenced the structure and controllability of the resultant holes in the starch foams.

Chemical Activation

Compared with physical activation, chemical activation is the preferred method to obtain high-performance porous carbon from different materials. The most commonly used chemical activation agents are KOH, NaOH, H₃PO₄, ZnCl₂, K₂CO₃, and Na₂CO₃. During the preparation of supercapacitors from cationic starch using KOH, ZnCl₂, and ZnCl₂/CO₂ activation, hysteresis is mainly due to factors such as the asymmetric slit-shape pores and the presence of ink bottle-type pores. In addition, KOH-activated starch is quite different to the others in terms of its surface morphology characteristics, as it has a number of shallow concave round pores on the surface, which may be suitable for ion mobility and storage. H₃PO₄ is the preferred chemical activator as the activation conditions are milder. H₃PO₄ can be recovered, and the corresponding porous carbon has a high yield and well-developed pore structure. When starch was directly impregnated with H₃PO₄, starch particles assembled together, and H₃PO₄ interacted with them to form phosphate and polyphosphate bridges. The pores were generated through dehydration reactions between acid molecules upon heating. The pore structures were created by the insertion of phosphate groups during the dilation processes.

High concentrations of acid and base are usually used to destroy the intermolecular hydrogen bond interactions and crystallization regions, which then facilitates the chemical reaction between the starch and modifiers. A base can weaken the intermolecular interactions of the starch molecules and facilitate the reaction between starch and modifiers. When the modifier/starch mole ratio was increased, the porous structure was maintained, and when the NaOH concentration was increased to a high level, the porous structure was destroyed and disappeared.

Direct Carbonization

To circumvent the disadvantages of an inert atmosphere to eliminate non-carbon elements, high-energy consumption and corrosion-resistant equipment have been used. In the study by, a series of porous carbons were prepared by two distinct synthesis methods, including a direct solid-state method plus hydrothermal treatment and the post-annealing method. The outcomes of these two methods were compared. The charge storage mechanism for the porous carbon can be described as a double-layer model, which is a reversible ion adsorption process on the carbon surface. The charge is reserved in the interface between the carbon electrode and the electrolyte, thus leading to the so-called double-layer capacitance. Porous carbons are produced by carbonization of the precursor in an inert atmosphere to eliminate non-carbon elements, followed by activation of the char with an activation agent to create the porous structure.